This invention relates to a process for forming pressure sensitive polymeric microspheres, and to the solid microspheres so formed as well as their use as repositionable pressure sensitive adhesives.
Solid and hollow inherently tacky acrylate polymer microspheres are known in the art for use in repositionable pressure sensitive adhesive applications. The term “repositionable” refers to the ability to be repeatedly adhered to and removed from a substrate without substantial loss of adhesion capability.
U.S. Pat. No. 3,691,140 to Silver teaches preparation of such microspheres utilizing water soluble, substantially oil insoluble ionic monomers as components of the acrylate polymer. Amongst others of such monomers are disclosed sodium acrylate, sodium methacrylate, and ammonium acrylate. Subsequent U.S. Pat. No. 5,053,436 to Delgado discloses that the Silver '140 patent teaches that such water soluble, substantially oil insoluble ionic monomers are critical to preventing coagulation or agglomeration of the formed microspheres.
However, more recent U.S. Pat. No. 5,656,705 to Mallya et al (col. 6, “Comparative Study and (Controls 1 to 4)”) in duplicating the procedure of the Silver '140 patent using sodium acrylate as ionic comonomer, found that during the polymerization essentially all the sodium acrylate was unreactive. Decreasing the level of such sodium acrylate led to reactor fouling—i.e. the instability probably ionic monomer helps to avoid.
It would be desirable to use such ionic monomer for the suspension stability function taught in the noted art without leaving a substantial quantity as unconverted monomer dissolved in the aqueous phase after polymerization. High levels of unreacted residual monomer are undesirable and, depending on monomer identity, can lead to downstream handling problems. For example, ionic acrylic acid (e.g. NH4 acrylate) is hazardous to humans and quite odorous and corrosive.
With respect to microsphere loss during repositioning, microspheres are transferred from the carrier or backing layer to the substrate. When microspheres are pulled away from the carrier or backing layer, this is recognized as a post synthesis application problem in the art. Low or no microsphere transfer is desirable in order that the carrier or backing layer be able to stick to another surface after repositioning.
Many approaches are disclosed in the art to minimize microsphere adhesive transfer. These include use of special emulsifiers during a two step polymerization procedure to promote formation of hollow microspheres; use of special fine particles interspersed with the microspheres; coating the microspheres with a separate adhesive layer; use of a special adhesion-promoting co-monomer during polymerization formation of the microspheres; use of a binder material providing sockets in which the microspheres are mechanically held; use of a primer applied to the carrier or backing layer, etc.
A further need continues to exist in the art for techniques to improve the adhesive transfer performance property of inherently tacky acrylate pressure sensitive adhesive microspheres.